Identification of bacterial contamination in food often occurs subsequent to an outbreak of a foodborne illness. Bacteria of Salmonella enterica subsp. enterica serovars are frequently identified as a food contaminant of many foodborne illnesses.
Ingestion of S. Heidelberg and S. Hadar can result in Salmonellosis. Salmonellosis is characterized by diarrhea, fever, and abdominal cramps that typically arise 12 to 72 hours after infection. Salmonellosis usually lasts 4 to 7 days, and most people recover without treatment. In some cases Salmonella infection can enter the bloodstream leading to sepsis and can ultimately cause death if not treated. Children under the age of 5 years are most likely to get Salmonellosis. Children, the elderly, and immunocompromised are most likely to have severe infections.
S. Hadar infections have been linked to catfish, sliced deli meats, turkey burgers, and handling of live poultry (e.g., chicks, ducklings, turkeys)
S. Heidelberg infections have been linked to eggs, chicken, pork, salmon, and cheddar cheese. Recently, S. Heidelberg has been linked to outbreaks in ground turkey, and Kosher broiled chicken livers. A routine inspection by the FDA recently (May, 2012) detected S. Heidelberg at a large Iowa poultry house used for egg production. S. Heidelberg in egg layers are a concern since S. Heidelberg can infect eggs via the transovarian route in a manner similar to Salmonella Enteritidis, and S. Heidelberg has caused several egg associated outbreaks resulting in human illness and death. Eating eggs outside the home was identified as a primary risk factor for illness due to S. Heidelberg infection.
S. Heidelberg is one of the five serotypes most frequently isolated in human cases of salmonellosis in the U.S. The rise in prevalence of this serovar has been accompanied by increasing antimicrobial resistance. S. Heidelberg appears to be a highly clonal serovar, with 34 of 36 global isolates in the Salmonella enterica MLST database having an identical sequence type (ST15), and the other two (one from the US, and one from Australia) differing from this canonical serotype at only a single locus. A S. Heidelberg organism, if consumed in contaminated food, causes salmonellosis which is often characterized by fever, vomiting, diarrhea, abdominal cramps, and severe dehydration.
A PCR and gel-based molecular serotyping method for the detection and identification of S. Hadar, S. Heidelberg, S. Enteritidis, and S. Typhimurium was designed based on detecting the combination of O-antigen, phase 1 flagellar antigen (H1) and phase 2 flagellar antigen genes (H2). The O: H1: H2 assignment for S. Hadar and S. Heidelberg is C2: z10: e,n,x and B: r: 1,2, respectively. However, because other Salmonella enterica serotypes also carry each of these genes, this method is not sufficient to differentially identify a single serotype due to cross-reactivity. In some cases a single base pair difference in the flagellin gene sequence results in an amino acid change that displays an epitope with a different serotype. The developers of this test report that the H2 allelotyping primers cannot distinguish serovars Heidelberg (B: r: 1,2) from Bradford (B: r: 1,5), Winneba (B: r: 1,6), or Remo (B: r: 1,7), and cannot distinguish serovar Hadar (C2: z10: e,n,x) from Glostrup (C2: z10: e,n,z15).
Some S. Heidelberg-specific assays described in the literature are molecular serotyping assays based on a set of four multiplex PCR reactions targeting five O-antigen loci, the fliC gene, and the fljB gene. Although very specific, the complexity of this assay makes it unsuitable for specific detection of S. Heidelberg. Another multiplex test known is an assay that detects both S. Heidelberg and S. Typhimurium performed in combination with another assay that only detects S. Typhimurium. A positive signal from the first, but not the second assay indicates the presence of S. Heidelberg. Two step assays are always time consuming and can lead to errors.
Assays for rapid, sensitive, and specific detection of Salmonella enterica pathogens are extremely important from both a public health and economic perspective. There exists a need for novel assays and methods for detecting and differentiating various serovars of Salmonella enterica subsp. enterica such as S. Hadar and S. Heidelberg from other Salmonella enterica serovars to test food and other contaminated samples. Some potential uses would be identification of a pathogen in a contaminated food to identify a contaminant; and identification of a pathogen in a clinical sample to make a differential diagnosis of what microbe is causing a particular disease.